1. Field of the Invention
The present invention relates to a radiographic imaging apparatus for detecting a radiation image of an object, and a control method for the radiographic imaging apparatus.
2. Description Related to the Prior Art
An X-ray imaging system is known in a medical field in use of radiation, such as X-rays. The X-ray imaging system includes an X-ray generating apparatus and an X-ray imaging apparatus. The X-ray generating apparatus has an X-ray source for generating X-rays. The X-ray imaging apparatus detects an X-ray image of image information of an object by receiving X-rays transmitted through the object after emission from the X-ray source. The X-ray source is provided with an imaging condition inclusive of a tube current and a tube voltage, the tube current determining a dose of X-rays per unit time, the tube voltage determining energy spectrum of X-rays. The imaging condition is determined for each event of imaging according to a body part, age and the like of the object or a body of examination with X-rays. The X-ray source emits X-rays according to the imaging condition.
A newly suggested type of the X-ray imaging apparatus includes an FPD (flat panel detector) or image detector in place of X-ray film or imaging plate (IP) used conventionally (See U.S. Pat. Nos. 6,967,332, 7,235,789 and 7,507,970 (corresponding to JP-A 2004-130058)). The FPD includes a detection panel and a signal processing circuit. The detection panel has an imaging area including plural pixels and signal lines. The pixels store a signal charge according to a radiation dose of X-rays. The signal lines read the signal charge in connection with the pixels. The signal processing circuit reads the stored signal charge form the pixels as a voltage signal, and converts the voltage signal into image data of a digital form. Thus, the X-ray image can be viewed immediately after the imaging in the X-ray imaging apparatus including the FPD.
In the detection panel, each of pixels in the imaging area is constituted by a photo diode as a photoelectric conversion element, and a TFT (thin film transistor). Scintillator (phosphor) is provided in the imaging area for converting X-rays into visible light. The TFT is a switching element for turning on and off electric connection between the photo diode and a signal line, to change over operation of the pixel. When the TFT is turned off, a non-conductive state is created between the photo diode and the signal line, to start a storage step in which a signal charge is stored in the photo diode. When the TFT is turned on, a conductive state is created between the photo diode and the signal line, to start a readout step in which the signal charge is read from the photo diode through the TFT and the signal line.
It is necessary with the X-ray imaging apparatus having the FPD to perform control of start synchronization to start the storage step in synchronism with the emission start of X-rays, unlike the X-ray film or imaging plate (IP). A widely used example of the control of start synchronization is a signal communication method in which a sync signal is sent between the X-ray generating apparatus and the X-ray imaging apparatus.
Examples of the control of start synchronization include not only the signal communication method but an auto-detecting method disclosed in U.S. Pat. Nos. 6,967,332, 7,235,789 and 7,507,970. In the auto-detecting method, changes in the radiation dose of X-rays emitted by the X-ray generating apparatus are monitored in the X-ray imaging apparatus, to detect a time point of an emission start of X-rays in a manner of auto-detection. The X-ray imaging apparatus disclosed in U.S. Pat. Nos. 6,967,332, 7,235,789 and 7,507,970 includes detecting elements, disposed in addition to normal pixels in the imaging area of the FPD, for detecting the radiation dose of radiation to check a time point of the emission start of X-rays. The control of start synchronization is performed by use of the detecting elements in the auto-detecting method. It is possible in the auto-detecting method to perform the control of start synchronization without transmission of a sync signal between the X-ray generating apparatus and the X-ray imaging apparatus.
Also, U.S. Pat. Nos. 6,967,332, 7,235,789 and 7,507,970 disclose the use of the detecting elements for AEC or automatic exposure control instead of the use for the control of start synchronization.
In the AEC, a total radiation dose of X-rays received from the X-ray generating apparatus is measured by the X-ray imaging apparatus. The AEC is a control of an exposure of the X-ray image by stopping emission of X-rays according to sending of a stop signal to the X-ray generating apparatus upon reach of the total radiation dose to a predetermined threshold. The AEC is performed for suitably controlling the total radiation dose of X-rays. The AEC makes it possible to prevent drop of image quality as an optimum exposure is ensured. Also, overexposure to the object can be prevented. Even in use of the X-ray film or imaging plate (IP) distinct from the FPD, the AEC has been performed in the prior art by combining the X-ray film or imaging plate (IP) with an exposure control device referred to as a photo timer. As disclosed in U.S. Pat. Nos. 6,967,332, 7,235,789 and 7,507,970, the detecting elements in the FPD are used for the AEC to make a special exposure control device unnecessary in a form discrete from the FPD.
JP-A 2008-132216 discloses the X-ray imaging apparatus in which the signal communication method and the auto-detecting method of JP-A 2008-132216 can be used as methods for the control of start synchronization. The X-ray imaging apparatus of JP-A 2008-132216 includes a wireless communication function for wirelessly transmitting a sync signal in cooperation with the X-ray generating apparatus. In a normal situation, the control of start synchronization is performed in the signal communication method. If a communication state of the wireless communication becomes poor, or if failure in the wireless communication occurs, then an emission start of X-rays is detected with the FPD in the auto-detecting method, to perform the control of start synchronization. In short, the X-ray imaging apparatus of JP-A 2008-132216 utilizes the signal communication method for the control of start synchronization normally, but utilizes the auto-detecting method exceptionally in the case of the poor communication state.
In medical facilities with the X-ray imaging system, there has been a recent trend of changing over from a conventional type of the X-ray imaging apparatus with the X-ray film or imaging plate (IP) to a new type of the X-ray imaging apparatus with the FPD. However, the entirety of the X-ray imaging system is remarkably expensive. A cost of updating is seriously high if the X-ray imaging system inclusive of the X-ray generating apparatus is totally updated. Thus, there is an idea of paying for introducing only the X-ray imaging apparatus with the FPD, and combining this with the existing type of the X-ray generating apparatus to update the X-ray imaging system.
As described heretofore, the control of start synchronization is required between the X-ray imaging apparatus and the X-ray generating apparatus to use a new type of the X-ray imaging apparatus having the FPD. A known type of the X-ray generating apparatus has the communication function in connection with the X-ray imaging apparatus, and a communication interface (standards of a cable and connector, signal format, and the like) is compatible with the communication interface of the X-ray imaging apparatus. In the case of this communication compatibility between the X-ray generating apparatus and the X-ray imaging apparatus, it is possible to perform the control of start synchronization in a normal type of the signal communication method.
In general, the signal communication method is more normally used than the auto-detecting method, and is more reliable than the latter as a method of the control of start synchronization. In the case of communication compatibility with the X-ray generating apparatus, it is preferable to perform the control of start synchronization of the signal communication method in the X-ray imaging apparatus.
However, it is likely that the X-ray generating apparatus of the existing type does not have the communication function for communicating with the X-ray imaging apparatus. Even if the communication function exists, communication incompatibility is likely to occur between the communication interface of the X-ray generating apparatus and that of the X-ray imaging apparatus. Communication is impossible between the X-ray generating apparatus and the X-ray imaging apparatus, in which the control of start synchronization according to the signal communication method cannot be performed.
For this situation, the X-ray imaging apparatus in which the control of start synchronization of the auto-detecting method is possible according to U.S. Pat. Nos. 6,967,332, 7,235,789 and 7,507,970 and JP-A 2008-132216 can be used, so as to establish the X-ray imaging system in combination of the X-ray generating apparatus of a conventional type.
The AEC is a control on a condition of stopping emission of X-rays by sending a stop signal from the X-ray imaging apparatus to the X-ray generating apparatus. As emission of X-rays cannot be stopped in the case of impossibility of communication with the X-ray generating apparatus, effect of ensuring an optimum exposure of the X-ray image, effect of overexposure to the object, and other effect of the AEC cannot be obtained. Specifically, if the AEC is performed in the X-ray imaging apparatus without communicability with the X-ray generating apparatus, no stop of emission of X-rays occurs, because the X-ray generating apparatus cannot receive a stop signal even upon outputting the stop signal from the X-ray imaging apparatus. On the other hand, it is likely in the X-ray imaging apparatus that the readout step is started after the storage step on a condition of stopping emission of X-rays with the AEC. As the emission of X-rays continues even during the readout step, noise may be caused to lower image quality of the X-ray image. Also, X-rays continue being applied even after the end of the storage step. X-rays not contributing to the X-ray image are emitted. Thus, no effect of preventing overexposing the object can be obtained.
Consequently, the combined use of the X-ray imaging apparatus with the X-ray generating apparatus can be made appropriate in relation to the control of start synchronization and the AEC of the X-ray imaging apparatus according to communication compatibility or incompatibility with the X-ray generating apparatus.
Although U.S. Pat. Nos. 6,967,332, 7,235,789 and 7,507,970 disclose both of the control of start synchronization and the AEC for the auto-detecting method by use of the detecting elements of the FPD, there is no disclosure as to which of the control of start synchronization and the AEC should be performed by use of the detecting elements.
Although JP-A 2008-132216 discloses the signal communication method and the auto-detecting method in relation to the control of start synchronization, there is no suggestion of the AEC.